<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>

  <script>
    function getSum(n = 0, m = 0) {
      let sum = 0
      for (let i = n; i <= m; i++) {
        sum += n
      }
      document.write(sum)
    }
    let num1 = +prompt("请输入开始计算的数字")
    let num2 = +prompt("请输入结束计算的数字")

    getSum(num1, num2)
  </script>
</body>

</html>